FIG. 1 shows a dual-supply amplifier 10. The two supplies to the amplifier 10 are positive (+VDD) and negative (−VDD). An input signal is supplied to the amplifier 10 to be amplified, and the amplifier 10 outputs an output signal to a load 20.
One common problem with such amplifiers, when used in audio applications wherein the load is a speaker, is the presence of an audible ‘pop’ when the amplifier switches on or off. Such pops are caused by a DC offset in the signal output from the amplifier; that is, the speaker outputs a noise i.e. an audio artefact, when rising to or falling from the DC offset to ground during turn on and turn off. Not only are pops irritating to the end user of audio equipment, for example, their associated DC offset is also a cause of power wastage. This is particularly true for portable audio players, where power consumption is an important consideration in order to maximize battery life.
There are two main sources of DC offset in the output signal that may result in an audible artefact, i.e. a pop, being output to a speaker. One offset is inherent within the input signal, and the other offset is inherent within the amplifier itself.
In the amplifier 10 illustrated in FIG. 1, the input signal related offset is removed by using an AC coupling capacitor 30 i.e. a DC blocking capacitor. Such coupling capacitors are off-chip in integrated circuit audio circuits since they have large capacitance values, typically 2.2 μF, that are not realistically susceptible to integration due to the amount of area required to achieve such a high capacitance value. Thus, prior to the amplifier 10, a large-value capacitor 30 is placed in the playback, i.e. signal, path to prevent any DC component in the input signal from being input to and then output from the amplifier 10. This solution, however, requires pins for connecting the external AC blocking capacitor 30 to the integrated circuitry as well as associated “real estate”, i.e. die area, on the chip. Two pins 32, 34 are required to connect the off-chip capacitor 30 to the on-chip circuitry. Most modern integrated circuits have stringent design constraints as to pin count, die area, package requirements, external components and the like, as such design parameters all add to system costs. Therefore, extra pins and external components are disadvantageous.
The amplifier 10 offset, and indeed any input signal offset, may be removed, or at least minimised, by using an analogue feedback circuit which acts to remove any undesired offset voltage at the input to the amplifier. The analogue feedback circuit is arranged to compare the amplifier output voltage to a target output bias, or reference, voltage and to feed back a corresponding voltage to be combined i.e. either added or subtracted, with the input signal. The realisation of low frequency time constants associated with audio applications requires the use of high resistor or capacitor values which for discrete solutions poses relatively minor problems. However, in order to realise an integrated analogue feedback circuit solution the use of high resistor and capacitor values is prohibitive due to area and process tolerance restraints.